The present invention is a method for enhancing the adhesion of a polymer mixture which includes a carbon black filler by adding a second filler. The present invention also relates to compositions having enhanced adhesion which include a polymer mixture, a carbon black filler and a second filler.
Carbon black fillers are frequently added to both rubbery and glassy polymers to increase the modulus and/or the fracture toughness. A major disadvantage of these filled composites is a significant reduction in their adhesion strength with other materials. This makes them difficult to use in multi-component structures, such as rubber tires, where many different types of carbon filled rubbers are layered together. Typical concentrations of carbon black fillers added to tire rubber can be in excess of 30% by weight. The addition of commercially available carbon black fillers to rubbery polymers such as polyolphins (i.e., polybutadiene, brominated isobutylene methylstyrene) and glassy polymers (i.e., polystyrene, and polymethylmethacrylate) can drastically reduce interfacial adhesion. This can cause catastrophic failure in objects composed of multi-component polymer layers. An example is the separation of the tread or the inner liner from other components in rubber tires. The failure is attributed to reduced adhesion which is possibly caused by pinning of polymer chains on the carbon black particle surfaces.
Polymer filler composites are widely used in many applications to provide such properties as hardness, heat resistance and electrical conductivity, simply by varying the filler concentration. In rubbers and other visco-elastic materials, the increase in modulus, (E−Eo), is related to the filler concentration, Ø, by the Guth Gold Equation.E−Eo=(2.5ØEo)  (1)
For example, in order to achieve optimal strength, rubbers in tires have a filler fraction in the range of 20-30% by volume. In glassy polymers, inorganic fillers are known to deflect cracks, hence increasing the fracture toughness.
The present invention overcomes the problems in the prior art using a simple and inexpensive method that can reverse the adverse effects which carbon black fillers have on polymers without affecting the increased modulus provided by the carbon black filler.